Comet Hale-Bopp Holds Clues to Creation of Comet Ices

AMHERST, Mass. -- University of Massachusetts astronomer
Matthew Senay is part of a research team reporting that Comet
Hale-Bopp, which passed by the Earth last year, provides
evidence that comet ices are created inside huge interstellar
clouds of gas, dust, and ice. The findings, detailed in the
current issue of Science, suggest that comet ices are not, as
previously thought, created in the outer regions of the solar
system, where comets themselves are formed. The study also
offers new evidence which implies that the temperature of the
interstellar cloud that formed our solar system could not have
been colder than 30 degrees Kelvin; a temperature considerably
higher than scientists previously believed. The research team
includes astronomers from the University of Hawaii, the
National Research Council of Canada, and the Observatoire de
Paris-Meudon.

Astronomers studied the chemical composition of Hale-Bopp's
nucleus during its journey past the Earth in April of 1997,
and compared their findings with the chemistry of interstellar
clouds, which are comprised of gas and ice-mantled dust. These
clouds can become massive, fragmenting and collapsing into
smaller clouds or "protostellar nebulae." Chemical reactions
within the smaller clouds cause significant amounts of
deuterium, a form of hydrogen sometimes called "heavy
hydrogen," to latch onto the ice-encrusted dust particles. An
individual protostellar nebula continues collapsing until the
beginnings of a star appear at the center, with planets
forming in the flattened disk of gas, dust, and ice -- called
the "protoplanetary nebula" -- around it.

Using a radio telescope located in Hawaii, astronomers
determined the ratio of deuterium to hydrogen in the hydrogen
cyanide molecules ejected by Hale-Bopp. These molecules cannot
be seen visually; however, they emit radio waves at specific
frequencies, enabling scientists to measure molecular
abundances with precision, Senay says. The team then compared
this ratio with the ratio of the same substances in the
hydrogen cyanide of interstellar clouds. The large deuterium
to hydrogen ratio in the hydrogen cyanide ejected from the
cometary ices of Hale-Bopp is evidence for interstellar ices
being the forerunner of comets, Senay says.

This research provides evidence that cometary ice is created
within interstellar clouds that have not yet collapsed; the
ice is then incorporated into the comet during its formation.

Editor's Note: Matthew Senay may be contacted at 413/545-4261
or senay@fcrao1.phast.umass.edu